Hitherto known apparatuses for production of microfibers and nanofibers working on principle of electrostatic field of very high intensity, the effects of which form melt or solution of polymers into fibrous structures, use plate collecting electrodes most frequently. The first methods of polymers spinning have been patented as far back as at the beginning of the 20th century U.S. Pat. No. 0,705,691 (1900), U.S. Pat. No. 0,692,631 (1902), U.S. Pat. No. 2,048,651 (1934) as reported in Recent Patents on Biomedical Engineering 1, 68-78 (2008), by Kumbar et al. Individual fibers deposited onto such a plate electrode are placed at random, i.e. they are not placed in any preferred direction. It is because of an unstable phase of a moving polymer jet, the trajectory of which is very complicated and spatially chaotic before its incidence onto the collecting electrode.
If the material produced is composed of regularly arranged microfibers or nanofibers, applications of such materials can spread boundlessly also in many new modern fields and branches. Their promising potential consists in substantial improvement of their morphological properties and consequently mechanical, physiological, biological, physical, optical and chemical properties, namely in particular thanks to their internal regularly oriented structure.
Several publications deal with principals of providing the arrangement of fibers deposited in this way. Two basic methods are known. The first one utilizes a mechanical principle of winding fibers onto a cylinder, bar or disc, rotating at high revs. The second principle, which this invention also refers to, utilizes static gathering collector divided into two or more conductive parts, separated from each other by a non-conductive gap of a definite size. The collector shapes the lines of force of an acting electrostatic field. The trajectory of the polymer jet is determined by these electrostatic forces and fibers falling onto the gathering collector are deposited parallel to each other in preferred direction in the non-conductive areas of the divided collector. The structure of the conductive and non-conductive areas of the collector defines the acting electrostatic forces, influencing hitherto random flight of the polymer jet, and thus it controls its movement. The mechanism of the ordered depositing of fibers onto the collector can be deduced from systematic experimental studies or numerical simulations of a physical model. In principal these methods work successfully. In 2003-2005, Dan Li et al. published the principle discussed above in professional journals, including Nano Letters 3 (8), 1167-1171 (2003), Advanced Materials 16 (4), 361-366 (2004), and Nano Letters 5 (5), 913-916 (2005).
The production of planar (2D) or voluminous (3D) materials using similar apparatuses is significantly limited and it is not possible to produce larger 2D and thicker 3D materials having regular structure. Thus the production is restricted to manufacturing of individual oriented fibers only. Ordered micro- or nanofibers are deposited onto non-conductive areas of the divided collector, where they form a fine regular layer. The divided collector consists of conductive usually metallic links separated by non-conductive backplate having high resistivity (higher than 1016 (Ω·cm). Fibers deposited onto such gathering collector are mechanically connected with it, so that any further independent practical use of them is limited. Positioning of an underlying substrate on the divided collector, or rather between emitter and collector, leads to a degradation of the structured electrostatic forces, the effects of which take part in the formation of fibers orientation. For an application of materials produced by this method, the resulting layer has to be taken from the collector first and transferred.
Rouhollaha Jalili et al. (Journal of Applied Polymer Science 101 (6), 4350-4357 (2006)) describe a simple collector for an accumulation of several oriented fibers into a common bundle. The result of it is not a planar structure but the bundle of fibers, only. Such fiber sample was prepared solely for the purpose of subsequent X-ray and mechanical analyses of the bundle properties. Practical use of the several fibers bundle is not mentioned in Jalili et al., and due to the achieved dimensions (length of 30 mm and diameter of about 0.08 mm), it may be assumed that it is not significant.
Patent applications US2005-0104258A1 and PPVCZ2007-0727A3 discuss a collecting electrode structure generating singular charges, but they do not deal with any ordered formation and orientation of fibers. A divided collector is a part of a U.S. Pat. No. 4,689,186, but it is used for different purposes and it is not directly involved in any formation of oriented fibers. Patent application EP2045375A1 describes an apparatus for production of 2D or 3D materials composed of micro- or nanofibers with regular structure using an electrically divided collector of cylindrical shape, during a rotation of which oriented fibers are collected. By means of the described solution it is possible to produce materials with a restricted dimension that is partly limited by the diameter of the rotating collector. Also an implementation of the apparatus for producing materials of this type with larger area (i.e. multiple repeating of the proposed solution) is practically complicated, line restricted and therefore ineffective.
Micro- or nanofibers of lower strength, especially fibers made of biopolymers, are being torn by their own gravity between the collector electrodes when thicker layers (2D or 3D) are to be formed and thus the whole structure is being impaired. This is limiting for any production technology and for getting applicable materials having desired parameters.
When depositing fibers in thicker layers, a degradation of an orientation level occurs and fibers arrangement becomes more random again. It is caused by a progressive increase of electric charge in the formed layers of fibers, i.e. in those collector parts that should remain non-conductive and without electric charge, to enable correct functioning of the fiber orienting principle. This negative effect brings about depositing of oriented fibers in lower layers of material only, i.e. in those layers which were deposited first at the beginning of the deposition; on the other hand fibers with random arrangement prevail in the higher layers. For that reason a structure of a gathering collector and an automatic mechanism were designed, where the automatic mechanism withdraws thin deposited layers of micro- or nanofibers and superimposes them in thicker layers (2D or 3D) simultaneously with the spinning process.